drm/xe/guc: Return CTB HXG response DATA0 if no buffer provided

Most of the synchronous GuC HXG action responses are defined in
such a way that only mandatory DATA0 from the HXG header is used
and only in few cases it is more than MBZ (must be zero).

For those cases where HXG action returns just DATA0, return that
value if caller didn't provide buffer for the full response.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240112102554.761-1-michal.wajdeczko@intel.com